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11: Physical layer - Encoding and signaling part 2



 Encoding and signaling
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Encoding  and signaling are the main functions of layer one of the OSI reference model, physical layer. At the data-link layer, frames are in the form of bits (zeros and ones), but when they get down to the physical layer, they get turned into other formats that the physical layer understands ; in this case they are transformed into signals, because they need  to be carried through cables and devices, which reside at the same layer, hence encoding and signaling. See the the following figure.


In order for data to be recognized by both sender and receiver, it must be put in a specific order, or grouped, or encoded into patters. Devices must  adhere to some rules when encoding data so that when it is received at the destination, it would be easily understandable by means of being subjected to the same rules, or decoding process. Thus, data is accumulated at the data-link layer as a frame, encoded into patterns recognized by layer 1 devices,  travels as signals onto media (e.g. cables, wireless), then received by the layer 1 of the destination and decoded  so as to be handed up to the upper layer as a frame again.

In addition to grouping data bits into patterns, encoding has got another function which is control information. As you know, a cable, on which bits in the form of signals are transmitted, is busy with signals containing zeros and ones, and to distinguish between what bits are the actual traveled data and what bits are non-wanted noise is very difficult ; so a method to tell the difference is required. For that reason, control information is utilized. This information is in the form of zeros and ones that indicate where a frame start and where the frame ends. The following figure clarifies how frames are distinguished from other extra bits.


Therefore, frames are put in a string of binary bits, comprising the control information that informs where the actual data is located within a stream of bits, as well as acting as a means of guiding the bits up to the destination by virtue of signals, whose function is to carry the  bits patterns. So signaling is another function of the physical layer.

Source machine and destination machine do the same thing in terms of encoding and signaling. When encoded and signaled frames get out the source machine heading towards the destination, this latter picks the frames in the form of signals and reverses the process. In other words, the destination apply the same mechanism to convert the patterns of physical energy into binary bits, then decoding the encoded bits to get the actual frame to be handed up to the upper layers.

Representing bits on media 

The methods of representing binary digits on physical media vary and covering all of them is beyond the scop of this study ; However, providing at least two methods, which are Manchester encoding and nonreturn to zero (NRZ),  to demonstrate how things are going on will suffice. These methods have their own distinctive ways of converting a pulse of energy into a defined amount of time referred to as a bit time.  This latter means how much time a given Network Interface Card at the OSI model takes generate 1 bit of data and put it onto media in the form a signal. So the way binary bits  are represented onto media  as a signal indicates whether it is 0 or 1 at a time. This is achieved by virtue of three possible variations which are amplitude, frequency, and phase. Take a look at this figure to have an idea.


Manchester encoding

This signaling method makes use of whether the amplitude is in a high or a low position.  If a voltage (amplitude) drops from  low to  high within the bit time, it represents a 1 ; whereas, in case a voltage  (amplitude) moves from  high to  low, it represents a 0. If a value is repeated such 1111 or 0000, it is represented by repeating the same movement of the amplitude. In addition, moving from one position to another (from high to low, or vice versa), happens at the edge of the time bit. The following figure shows how Manchester encoding occurs in a rather simplified way.


This signaling method is not suitable for higher-speed links. It is rather fitted for lower-speed links such as 10Base-T Ethernet whose speed is 10 megabits per second.

Nonreturn To Zero (NRZ)

NRZ stands for Nonreturn to Zero. It is another signaling method in which bits representation lies in the voltage level within a bit time. The level of the voltage can either be a 0 or a 1. More specifically,  If the the level of the voltage is high, then the voltage represents 1. Contrariwise, if the level of the voltage is low, or rather remaining steady, it is in this case a 0. Consider the following figure. 


But this signaling methods has also some weaknesses which prevent it from being used in higher-speed links. It is not inherently a self-clocking signal. Simply put, a string of 0's or 1's in NRZ prevent the sender and the receiver clocks from being synchronized.[post_ad]
11: Physical layer - Encoding and signaling part 2 Reviewed by BOUFTIRA on 10:07:00 AM Rating: 5

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